Electronic switching devices are generally connected upstream of electrical loads, for example upstream of a 3-pole motor load. Switching devices such as these in this case have three or else only two actually switched, that is to say controlled, phases. In this case, switching devices with two switched phases represent a technically financial optimum and are therefore enjoying increasing popularity on the market.
Each controlled phase in an electronic switching device is switched in its own right by at least one component, generally a power semiconductor. Components such as these produce heat losses which are often significant and must be dissipated via a heat sink. In many applications, it is also worthwhile and known for the component or the power semiconductor to be bridged in its permanently switched-on ON state by a mechanical switching element, so that the current flow is passed via the mechanical switching element and no longer via the power semiconductor. A so-called bypass circuit such as this makes it possible to drastically reduce the heat losses from a power semiconductor in the permanently switched-on state. Switching devices such as these therefore have a pairing comprising the semiconductor switching element and a mechanical switching element for each electrical phase.
Since switching devices are generally designed for high power levels, that is to say high voltages and currents, the individual phases must be arranged such that they are electrically isolated from one another. For this purpose, geometric separations must be complied with, which are dependent on the magnitude of the voltages carried by the phases. The higher the voltage, the greater the corresponding separation must be. For relatively low power level electronic switching devices, that is to say for example below 22 kW the power circuits and control circuits associated with each phase are arranged on one or more printed circuit boards. The aim in this case is always to optimally utilize the printed circuit board surface area but at the same time to maintain the safety separations mentioned above. As a result of the additional requirement that the semiconductor switching element must still additionally be thermally linked to a heat sink, a switching device often requires a large amount of physical space.
It is already known for a single heat sink to be provided for a switching device, and for all the semiconductor switches to be linked on one side to the heat sink. In this case as well, the isolation separations must be taken into account not only between the control circuits and load circuits but also between the corresponding individual phases, on the printed circuit boards to which the semiconductor switching elements are fitted. Switching devices such as these occupy a correspondingly large amount of space.
It is also known for each power section, which in each case corresponds to a single phase, to be arranged on a separate printed circuit board, for all the mechanical bridging switching elements to be arranged on a further printed circuit board, and for these four individual printed circuit boards to be connected to one another by soldering, together with a fifth printed circuit board which contains the control circuits. A design such as this is highly complex and results in additional connection and isolation parts.